1. Field of the Invention
The present invention relates to the field of non-dispersive infrared gas analyzers, and more particularly, to infrared non-dispersive gas analyzers, intended for everyday use in measuring the presence of one or more gases in a random gas sample.
2. Prior Art
Non-dispersive infrared gas analyzers are generally known in the prior art. These analyzers utilize an infrared source, typically an electric heater, to provide infrared radiation through a gas sample for detection by a detector. Various schemes have been utilized by these devices for providing the desired signal utilizing both single and double optical path configurations.
The fact that various gases exhibit infrared absorption characteristics for specific wave lengths of infrared radiation is well known, and such phenomena are at least theoretically easily usable for the detection of specific gases in a random gas sample. However, there are a number of practical problems which detract from the stability and sensitivity of such devices. By way of example, contamination on the windows of the sample cell will alter the radiation passing therethrough, which in most systems may be erroneously interpreted as the presence of the gas to be detected in the gas sample. Accordingly, many prior art systems required frequent renulling to cancel out the apparent zero offsets from this source. Furthermore, detector null shifts and IR source temperature changes would similarly provide a rather unstable zero output of these devices.
To minimize the IR source temperature shift, some prior art IR non-dispersive gas analyzers utilize a temperature controlled mounting for the IR source. Thus, the IR source is itself mounted on a base or platform which was heated to some fixed temperature above the normally expected ambient, and maintained at the temperature by some form of temperature control system. This system might be merely a mechanical thermostat for turning the power on and off, in which case, the temperature of the IR source will tend to cycle with the thermostat. Proportional temperature controllers are also used, which avoid this temperature cycling problem. However, such controllers are expensive since the amount of power being controlled may be as high as 1000 watts or more.
Prior art sample cells for retaining the gas sample in the optical path are generally closed cells having an inlet port adjacent at one end and an outlet port adjacent at the other end, and with infrared transparent windows at each end thereof. The sample gas flow through this type of sample cell is such as to encourage direct impingement of oils, particulate matter and other contaminates onto at least one of the infrared transparent windows, thereby tending to contaminate the window. This produces zero shift if the contaminant is a material to which the analyzer is sensitive and cuts down its transmissibility at a reasonably rapid rate resulting in deterioration of performance and ultimately cessation of proper operation. Minimizing window contamination in such sample cells is accomplished by rather fine gas stream filtration, resulting in higher maintenance effort and cost, and greater hangup due to the filter itself. To solve this problem, one prior art sample cell utilizes an air curtain in place of the transparent windows, wherein a flow of air passes each end of the sample cell, tending to contain the sample within the bounds determined by the air curtain. Thus, the window contamination problem is eliminated by elimination of the window itself. However, the air curtain system requires a separate air pump as well as other structure in order to accomplish the task, and tends to define a varying sample length thereby providing some uncertainty in the analyzer output. Furthermore, such sample cells are not enclosed, so that other components of the analyzer will be exposed to some contamination by the gas sample.
Prior art detectors typically are photoconductive devices mounted on a thermoelectric cooler with a thermister also mounted on the cooler. The entire assembly is mounted in a transistor can or other package with an infrared transparent window over the photoconductor. Such devices in cooperation with a temperature controller controlling the thermoelectric cooler in response to the thermister output are highly accurate and reliable devices provided the integrity of the hermetic seal on the can is preserved. If, however, only minor leaks occur, the inclusion of any moisture into the can usually causes rapid deterioration of the photoconductor, thereby providing a very unstable analyzer output and ultimately resulting in the total failure of the analyzer.
Prior art electronics for use with non-dispersive infrared gas analyzers are generally adapted to measure a detector signal and to provide an output in a manner consistent with the principal of operation of the analyzer. These electronic circuits have not provided a self test feature, etc., and typically required a large number of components to achieve the required task. The net result is that prior art analyzers tend to require zero ajustment, and periodic servicing and cleaning to maintain the devices in a satisfactory operation condition.